The goal of this work is to understand the molecular determinants underlying cell cycle control, which directly relates to many aspects of human health including cancer biology. In Caulobacter crescentus, the cell cycle is largely directed by two-component signal transduction systems consisting of sensor histidine kinases and their cognate response regulators. The Caulobacter genome contains a histidine kinase coupled to a photosensor-type LOV domain (hereafter known as LOV kinase). Transcription of this LOV kinase is cell cycle-controlled, peaking at the beginning of DNA replication. Thus, light may have a role in regulation of replication or some other aspect of cellular growth and development. I will approach this project via three routes: 1) characterize the photoactivity and spectral properties of LOV kinase in vitro and in vivo; 2) characterize the phenotype and expression profile of a LOV kinase knockout strain in the presence and absence of light; 3) identify other components of the LOV kinase cellular signaling network using a combination of affinity purification, mass spectrometry, and in vivo fluorescence resonance energy transfer (FRET).